1. Field of the Invention
This invention relates to graphics systems, and more particularly for a SVGA-to-TV scan converter integrated with a portable computer's graphics sub-system.
2. Description of the Related Art
Graphics standards for personal computers (PC's) have developed independently of standards for televisions. Thus a television monitor cannot receive computer-graphics data without a converter device to translate PC-graphics data into television picture data.
Converter devices known as scan converters are available for purchase by PC users. Scan converters are typically external devices which plug into the PC's graphics output port. The PC's graphics is converted to television format, either the National Television System Committee's NTSC standard or the Phase Alternation Line (PAL) standard common in foreign countries. The scan converters are often rather crude devices which lose a significant amount of the graphics data displayed on the PC's screen. Flicker can also be a problem.
FIG. 1 is a diagram illustrating the over-scan problem when translating PC graphics to TV. PC graphics display 11 displays many horizontal lines of pixels; five of these lines are shown in the simple example of FIG. 1. All displayable lines of computer-generated graphics occur in displayable area 14, which for PC's is typically set slightly smaller than physical screen area 12 so that all displayable lines are visible. Users can increase the horizontal and vertical span so that displayable area 14 touches the edge of physical screen area 12, but usually a small dark region 16 is present between displayable area 14 and the edge of physical screen area 12. Computer programs often place important graphics data near the edge of displayable area 14, so many PC users prefer to adjust displayable area 14 so that it entirely falls within physical screen area 12. Otherwise, the PC is difficult to operate, such as when the cursor is moved to the extreme edge of displayable area 14. The cursor would not be visible in such a case if displayable area 14 were larger than physical screen area 12, leaving the user to wonder where the cursor went to.
Televisions were originally designed to display video information from a broadcaster. Aesthetics were more important than viewing the entire broadcast image, so a dark region 16 such as on PC monitor 11 is unacceptable for television. To ensure that no dark region appears, a television image contains several over-scanned horizontal lines which fall outside physical screen area 24 and are not visible to the viewer. In FIG. 1, lines 1 and 5 are not visible since they fall outside of physical screen area 24. Lines 2, 3, and 4 are visible. Lines 1 and 5 are in over-scan area 18, falling outside physical screen area 24 but within broadcast image 22.
Television over-scan is a problem when converting PC graphics data to television format. Simply converting the pixels in the PC's displayable area 14 to the television broadcast image 22 causes lines in over-scan area 18 to not be visible. While just two lines are shown in FIG. 1 as falling in over-scan area 18, actual televisions have a large over-scan area 18 of dozens of horizontal lines. When the PC user moves the mouse cursor into over-scan area 18, perhaps by mistake, the cursor is not visible, leaving the user to guess where the mouse cursor went to.
The television standards require that the horizontal lines of graphics data be interleaved. Thus lines 1, 3, 5 . . . are sent to the television monitor as a first or even field, then lines 2, 4 . . . are sent as a second or odd field. Personal computer displays usually are non-interleaved, requiring that the lines be sent in order. Thus lines 1, 2, 3, 4, 5 . . . are sent to PC monitor 11 until all lines have been written. Scan conversion requires that the non-interleaved graphics from the PC be converted to interleaved data for the television.
PC displays are typically higher-performance displays than televisions, operating at roughly double the display rate of a television. Thus each frame displayed by the PC can be filtered to provide either the even or the odd field of the interleaved television. FIGS. 2A, 2B show that VGA lines from a PC can be selected, averaged, or filtered to produce half as many lines for one field of a TV image for the PAL standard. FIG. 2A shows that for the even field, VGA lines 1 and 2 are averaged together, perhaps on a pixel-by-pixel basis, to generate TV line 1. Likewise, VGA lines 3 and 4 produce TV line 3, and VGA lines 5 and 6 produce TV line 5. TV lines 2, 4, 6 are skipped during the even field.
FIG. 2B shows that for the odd field, VGA lines 2 and 3 are averaged together on a pixel-by-pixel basis to generate TV line 2. Likewise, VGA lines 4 and 5 produce TV line 4, and VGA lines 5 and 7 produce TV line 6. TV lines 1, 3, 5 are skipped during the odd field.
A simple ratio of two VGA lines for every TV line is needed for converting VGA to PAL-TV. The overall refresh rate of the TV is half of the VGA refresh rate, with one TV field being refreshed for each VGA screen refresh. Twice as many lines are being written to the VGA display as to the TV during any time period. The VGA timing may have to be adjusted slightly to match the PAL-TV timing standard for horizontal and vertical refresh rates. The pixel rates also differ; typically the number of pixels in each line are reduced as each line is converted.
SIMPLE 2:1 CONVERSION FOR VGA TO PAL-TV--FIG. 3
FIG. 3 is a diagram of the display and blanking periods for two VGA frames being converted to one PAL-TV frame. The VGA timing is set to have 625 lines, perhaps more than is otherwise required for a CRT or a flat-panel display, since only 480 lines of computer-generated graphics are displayed by the VGA standard. The PAL standard specifies exactly 625 lines for a frame; 313 lines in the even field, and 312 lines in the odd field. The extra line in the even field can be accounted for by adjusting the pipelining of the line conversion.
During even display period 40, 240 TV lines are written to the TV. These 240 lines are within the viewable area of the TV monitor. These 240 lines are generated from exactly twice as many VGA lines: 480, which are written to a PC monitor about one line-period before the corresponding line is written to the TV. During even vertical blanking period 42, the PC's CRT monitor and the TV monitor's picture tube are re-traced to the beginning of the first line from the end of the last line. This is a slow process, especially for TV, and 73 TV lines are required by the PAL specification. Although double 73 is 146, only 145 dummy VGA lines occur during this period, causing the pipelining of line to shift for the odd field relative to the even field.
During odd display period 44, 240 TV lines are written to the TV. These 240 lines are generated from exactly twice as many VGA lines: 480, which are written to a PC monitor about two line-periods before the corresponding line is written to the TV. During odd vertical blanking period 46, the PC's CRT monitor and the TV monitor's picture tube are re-traced to the beginning of the first line from the end of the last line so that the next even field can begin. The PAL specification requires 72 TV lines during odd blanking. Although double 72 is 144, 145 dummy VGA lines occur during this period, causing the pipelining of line to shift back for the next even field.
A constant ratio of two VGA lines for every TV lines can be used for conversion. However, standard VGA controllers display only 480 lines, and need relatively few lines during the vertical blanking period. Thus only 525 lines are normally used, rather than the 625 lines required for exact 2-to-1 conversion to PAL-TV. The PC user must use software to change the total number of lines from 525 to 625 for this conversion to operate properly. A 28.322 MHz pixel clock is assumed, with each line having a total of 906 pixels, including 640 displayable pixels.
2:1 CONVERSION TO NTSC-TV LOSES GRAPHICS DATA--FIG. 4
When a 2-to-1 line ratio is applied to the NTSC TV standard, some of the VGA lines displayed on the PC's monitor are not displayed on the TV. FIG. 4 highlights conversion from VGA to NTSC-TV with a loss of VGA data.
During even display period 41 only 200 TV lines are written to the TV for the NTSC standard rather than the 240 lines for the PAL standard. These 200 lines correspond to only 400 of the 480 viewable VGA lines. Thus 80 viewable VGA lines are lost when using 2-to-1 conversion for NTSC-TV.
During even vertical blanking period 43, vertical re-trace occurs. The NTSC specification requires 63 lines, corresponding to 125 VGA lines. Odd vertical blanking period 47 requires 62 lines.
During odd display period 45, 200 TV lines are written to the TV, and 400 VGA lines are written to the PC's display. The PC's vertical blanking period does not being for another 80 lines, when all 480 lines are written. Since these 80 VGA lines are converted to non-displayable TV lines in odd vertical blanking period 47, the graphics from these 80 lines are not displayed on the TV. Loss of graphics data is undesirable.
Other PC-graphics standards use a different number of lines, and conversion is more complex since the numbers do not match the TV standards as nicely as VGA does. Super-VGA (SVGA) is commonly used today, and some scan converters are available for SVGA. Unfortunately SVGA scan converters are expensive. While VGA scan converters can perform conversion using small memories containing one or two horizontal lines; SVGA converters use large memories which buffer an entire frame. Since a SVGA frame is 600 lines, the full-frame buffer is much more expensive than the line buffer.
What is desired is a scan converter which can convert PC graphics to TV formats. It is desired to convert SVGA graphics as well as VGA graphics, but without the expense of a full-frame buffer. It is desirable to use a line buffer rather than a frame buffer for scan conversion, even for SVGA. A method and apparatus for converting PC graphics to TV formats for arbitrary standards is desirable. It is desirable to use a line buffer for conversions with arbitrary ratios of PC lines to TV lines. It is also desired to exploit characteristics of portable PC's flat-panel displays for an integrated scan converter.